I. Field of the Invention
The present invention relates to the casting of steel strip. More particularly, the invention relates to a method and apparatus for the sequence casting of steel strip without the need for a new casting start.
II. Summary of the Prior Art
In the continuous casting of steel, interruptions of the casting operation are sometimes required. For example, wear of the pouring tubes, stoppers and tundish requires the performance of periodic maintenance upon and/or the replacement of these parts which cannot be accomplished while the casting operation is ongoing. Similarly, a change in the type of steel being cast requires an interruption in the continuous casting operation. The complete clearing of steel melt from the mold in such cases is undesirable, however, because each new casting start requires time-consuming and expensive preparatory work. Hence, in such cases, the art generally interrupts the continuous casting operation, keeping the end of the first melt in the ingot mold while the necessary maintenance or repairs are performed or the steel type of the melt is changed. Thereafter, a second melt is cast onto the end of the first melt in order to continue the casting operation.
Difficulties sometimes arise in the above regards because the connection between the trailing end of the first melt and the leading end of the second melt must be strong enough to allow the discharge means of the mold which pulls upon the portion of the first melt remaining in the mold to draw the leading end of the second melt through the forming area of the mold. If this connection is not strong enough to withstand the applied tension, the leading end of the second melt may be left behind in the mold. This condition is unsatisfactory, and may be time consuming and expensive to correct. Accordingly, in the sequence casting of steel slabs it has been found (Japanese Patent Application No. 57921/75) that it is advantageous to provide an improved connection between the slabs. In this case, after the melt feed is interrupted, an elongated member having an I-shaped cross section is partially immersed horizontally in the surface of the melt within the mold such that the web connecting the two shorter sections of the "I" is substantially parallel to the direction of travel of the melt and portion of its length extends upwardly from the surface of the melt. The surface of the melt is then covered with cooling material and brought to solidification. Resumption of the casting operation is thereafter accomplished by casting the start of the second melt about the portion of the I-shaped member projecting from the trailing end of the first melt, allowing the start of the second melt to solidify, and withdrawing the trailing end of the first melt from the mold. The locking engagement of the I-shaped member with the trailing end of the first melt and the leading end of the second melt allows the web portion to transmit the extraction force to the leading end of the second melt and draw it through the mold as well. Of course, once the start of the second melt reaches the mold discharge means, the continuous casting operation proceeds as usual.
The foregoing method is not suitable for sequence strip steel casting because an exact positioning of an appropriately smaller I-shaped member in the surface of the melt within the mold is not possible in the narrow confines of the molds used for strip casting. Further, the walls of ingot molds used for strip casting are constantly cooled. This results in a strand shell which rapidly grows inwardly from the mold walls. An I-shaped member would tend to become hung up at the lateral strand shell thereby preventing its horizontal immersion into the melt. Accordingly, the reliable creation of a connection between the first and second melts strong enough to withstand the applied extraction forces cannot be reliably assured by the foregoing method.